Electric motors are used in various use cases. One scenario in which an electric motor finds application are belt-driven starters and generators (BSG). Here, an electric motor is coupled with an internal combustion engine of a vehicle, e.g., via a crankshaft of the combustion engine.
A direct current (DC) power supply is used to operate the electric motor. Recently, use of DC power supplies having a DC supply voltage of 48 V has become popular.
Various implementations of electric motors are known. For example, implementation of electric motors in a claw-pole design is known. Generally, electric motors include a stator and a rotor, wherein the rotor can move with respect to the stator. An excitation coil is used to polarize magnetic material, to thereby increase the magnetic flux between rotor and stator. Thereby, a torque acting between the stator and the rotor can be increased.
Polarization of magnetic material corresponds to aligning the magnetization of the magnetic material along a common direction; such a state is sometimes referred to as saturation.
A current flow through the excitation coil is used to charge the excitation coil and to polarize the magnetic material towards saturation. The direction of the magnetic field in the excitation coil is the same as the direction of the magnetic field in the magnetic material. Typically, soft ferromagnetic material is used. Thereby, the polarization of the magnetic material can be varied significantly between saturation (full magnetic field applied) and remanence (no magnetic field applied), by charging and discharging the excitation coil.
The current flow in the excitation coil is a DC current. Generally, the current flow is not larger than 10 A. The excitation coil can be modelled by an inductor having a significant inductance, and a resistor connected in series with the inductor. Typical values of the inductance are a few mH to a few hundred mH.
A BSG can operate in different system states. A first system state is sometimes referred to as motoring state; and the second system state is sometimes referred to as generating state. For example, in motoring state, the electric motor drives a load, typically, the combustion engine; differently, in generating state, the electric motor is driven by the load, e.g., due to mass inertia thereof. As a general rule, the BSG will consume power provided by the DC power supply in motoring state; but will provide power to the DC power supply in generating state. For example, in motoring state, the BSG can provide a torque for start-up of the combustion engine or additional acceleration. Differently, the generating state may be activated when the combustion engine is not required to provide torque, as may be the case, e.g., during braking or freewheeling of the vehicle. In generating state, the BSG can act as an alternator to provide electrical energy to the DC power supply. For example, the electrical energy may be used for charging a battery connected to the DC power supply.
However, there is a risk that providing electrical energy to the DC power supply results in a failure state or even damage to the system. For example, scenarios can be encountered in which the battery is disconnected from the DC power supply. Then, the electrical energy provided by the BSG cannot be used for charging of the battery. Then, a voltage on the DC power supply may exceed a threshold voltage associated with safe operation. For example, a typical threshold voltage of a DC power supply operating nominally at 48 V may be 60 V. This threshold voltage can be reached quickly, e.g., if the torque provided to the BSG is high, e.g., as may be the case for high-speed operation that can be encountered at large velocities of the vehicle. If the threshold voltage is reached, breakdown of electric components including damage to the electric components can occur.
There are various techniques known in the art to mitigate such excessive feedback of energy into the DC power supply.
One technique involves reducing the current flow in the excitation coil, preferably to zero. Then, the magnetic material is de-polarized and the induced voltage at phase windings is reduced, due to weaker flux coupling. Typically, a design strategy of electric motors includes setting the voltage induced at the phase windings due to the phase windings moving relative to the magnetic field, the so-called “back electromotive voltage”, BEMV, sometimes also referred to as “back electromotive force” (BEMF) such that, at zero current flow through the excitation coil, a threshold voltage of the respective DC power supply is not exceeded. The BEMV is a voltage across any two motor phase winding terminals, which is generated in electric motors, when there is a relative motion between the armature and the magnetic field produced by the field coils of the motor. Since the BEMV has a tendency to increase with increasing motor speed (e.g., measured in rounds per minute, rpm), the design—e.g., of the size of the gap between rotor and stator, the shape and magnetization of the magnetic material used, etc.—is typically set such that this design constraint is fulfilled at all relevant motor speeds. For example, a 48 V BSG can be designed for a maximum motor speed of 16,000 rpm: here, the BEMV at maximum current flow—e.g., 4 A—in the excitation coil can amount to 250 V; differently, the BEMV at zero current flow through the excitation coil can amount to only approximately 50 V, which is well below the typical threshold voltage of 60 V. From this example, it is clear that even for small residual current flows in the excitation coil there is a significant risk of damage of electrical components due to overvoltage at the DC power supply. Typical time durations from maximum current flow to zero current flow in the excitation coil (discharge time) can be as long as 15 ms for worst-case scenarios. A discharge time in this order of magnitude is typically long enough to result in damage of electrical components due to excessive overvoltage fed back to the DC power supply.
To reduce the discharge time, flux weakening techniques (sometimes also referred to as field-oriented control (FOC)) can be employed. For example, a FOC technique is described in U.S. Pat. No. 9,614,473 B1 or Wai, Jackson, and Thomas M. Jahns, “A new control technique for achieving wide constant power speed operation with an interior PM alternator machine”, Industry Applications Conference, 2001, Thirty-Sixth IAS Annual Meeting, Conference Record of the 2001 IEEE, Vol. 2, IEEE, 2001. In FOC, a current vector angle is controlled. An inverter is employed. Typically, the effectivity of the FOC is limited by the output current ability of the inverter. For example, a maximum output current of a 6 phase inverter can be limited to 150 A rms for each phase. Typically, due to the limited output current ability of the inverter, FOC may be helpful to reduce the gap magnetic field generated by the excitation coil current to some degree, but it is typically not possible to fully reduce the combined gap magnetic field to zero using FOC. Then, the residual current flow in the excitation coil can be sufficient to cause excessive overvoltage is on the DC power supply.
Further, to reduce the discharge time, an active short technique may be employed. For such techniques, 3 high-side/low-side switches are turned on synchronously, to force stator windings short for a certain time duration. During this time duration, the inverter cannot generate any voltage, irrespective of the motor speed. After the time duration, the switches are turned off again this. After the time duration, the current flow in the excitation coil has reduced to zero; and hence, the BEMV is comparably small. However, the active short technique has the disadvantage that comparably large torque can be observed. Typically, the torque created by the motor will follow and meet the commanded torque from a central control unit. Then, when shorting, the torque may further increase. Further, the current flow through the switches can be high when shorting. Damage may result. A further disadvantage includes increased complexity for the inverter when entering a safe state; this is because the driver circuitry for the 3 high side/low-side switches must be kept active, even when entering a safe state in which further components are disabled.